1. Field of the Invention
The present invention relates to an air conditioning apparatus for use as a room cooler, a room heater, or a room cooler and heater in which use is made of a scroll compressor to compress a refrigerant and, more particularly, an air conditioning apparatus of this kind in which the scroll compressor is operated under the control of an inverter which is driven at a high carrier frequency to control the capacity of the scroll compressor.
2. Description of the Related Art
In an air conditioning apparatus of the type in which an electric motor for driving a refrigerant compressor is operated under the control of an inverter of variable operating frequency, it has been usual to control the capacity of the compressor by varying the operating frequency of the inverter to change the speed of the electric motor, depending upon the capacity required for the air conditioning apparatus. The inverter is arranged to convert a DC current (usually produced from a commercial AC line by using rectifying and smoothing means) into an AC current (in general, a three-phase AC current) of an operating frequency, by means of a power switching element which is turned ON and OFF at a frequency corresponding to the operating frequency, and to feed the converted AC current to the electric motor. In order to increase the speed of the electric motor correspondingly to the increase of the operating frequency under the same torque, it is necessary to increase the output voltage of the inverter in accordance with the increase in the operating frequency. For this purpose, a system may be used in which the output of the inverter is varied by varying the DC voltage from which the AC voltage is converted, but an inverter of a voltage PWM (pulse width modulation) type has been frequently used which is arranged to hold the DC voltage at a constant value and to chop the output AC voltage by a frequency which is considerably higher than the operating frequency so as to increase ON-duty ratio, that is, to increase the time-averaged output voltage with the increase in the operating frequency. The pulse width modulation is effected by comparing the given time-averaged output voltage with the carrier voltage of high frequency and turning ON and OFF the switching element at a required ON-duty ratio at said carrier frequency on the basis of such comparison.
In the conventional air conditioning apparatus using the inverter of the voltage PW type, the operating frequency of the inverter for driving the compressor has been 30 Hz to 90 Hz or 30 Hz to 115 Hz, while the carrier frequency of the inverter has been 1 KHz to 2 KHz
As an example of the apparatus of this kind, U.S. Pat. No. 4,736,595 may be referred to.
In the prior art as described above, the carrier frequency of the inverter for driving the motor is relatively low, such as 1-2 KHz, and consequently, an electromagnetic audible noise is generated from the motor owing to said carrier frequency and particularly in the case where the motor is operated at low frequency such electromagnetic noise forms an uncomfortable noise which may adversely affect the ambient comfort.
Furthermore, in the case where the motor is operating at a low frequency, the pressure for feeding oil in the compressor is decreased owing to slowing of the speed of the compressor, with the result that the oil fed to bearing parts of the compressor decreases, thereby causing wearing of the shaft of the compressor. In order to avoid such defect it is impossible to decrease the minimum operating frequency below the above-mentioned limit.